Diabetes has become a global epidemic. In the United States alone, 26.9% of the population aged 65 years or older suffers from the disease.
Foot ulcers are one of the most common complications in diabetics leading to hospitalization. 45-83% of the annual lower extremity amputations in the United States involve diabetics. The direct costs for the treatment of diabetes and its complications in the United States were more than $116 billion in 2007, among which the treatment of foot ulcers accounted for at least 33%.
Foot ulcers are the most common precursor to diabetes-related amputation. Foot ulcer prevention has become the focus of amputation prevention programs. The etiology of ulcerations in people with diabetes is commonly associated with the presence of peripheral neuropathy and unrecognized repetitive trauma.
Foot-care practitioners often use therapeutic shoes and insoles to redistribute the forces on the foot. The standard technique to evaluate their efficacy has been focused on pressure reduction, simply because in-vivo testing of shear in gait lab or clinics is not readily available. Even though peak foot pressures at the site of neuropathic ulceration have been identified as a significant risk factor for foot ulceration, elevated foot pressures are not strongly associated with predicting the development of foot ulcerations in people with diabetes and neuropathy.
The Receiver-Operating Characteristic (ROC) for neuropathic subjects with diabetes indicates that high foot pressures (>87.5 N/cm2) had a sensitivity of 63.5%, a specificity of 46.3%, a positive predictive value of 17.4% and a negative predictive value of 90.4%. This suggests that other pathologic forces on the sole of the foot, such as the shear forces, should be considered in the etiology and prevention strategy for foot ulceration.
In addition, pressure ulceration developed in superficial skins and deep tissues is a common problem encountered by diabetics, lower extremity amputees, wheelchair users, and bedridden patients. For example, 40-60% of the prosthesis users have skin problems.
A big challenge in addressing ulcers is the current lack of understanding of its etiology. It is well-established that the risks of ulceration are determined by the combination of two stress components, i.e. the pressure normal to the contact surface and the shear stress tangential to it. Even though peak foot pressures at the site of neuropathic ulceration have been identified as a significant risk factor for foot ulceration, elevated foot pressures are not strongly predictive of developing foot ulcerations in persons with diabetes and neuropathy.
Unfortunately, the role of the shear stresses is very complicated. On one hand, shear stresses are needed to prevent slippage. On the other hand, when combined with pressure, shear stresses are known to promote blood occlusion, a major cause of skin damages. Different shear stress magnitudes could also result in different types of skin damages. Small shear stresses cause rubbing that lead to skin irritation, abrasion, and/or blister formation, while large shear stresses, by preventing slippage, transfer the loads to the internal tissues.
As a result, excessive tissue deformation and stress concentrations at the tissue/bond interfaces can be generated and may eventually lead to deep tissue injuries. This is the reason why some researchers have suggested that the internal stress state instead of the superficial forces is a better indication of ulceration.
In order to calculate the internal stress state, however, both the pressure and the shear forces along the interface have to be known. Without a reliable and convenient way to measure the shear forces, scientific studies on the etiology of ulceration is severely hindered.
The availability of clinical pressure mapping system has advanced the understanding of skin ulcers and brought attention to the contribution of shear stresses. Unfortunately, the development of shear-sensing devices has been lagging behind that of pressure sensors. Despite multiple attempts in the past several decades, currently there is no shear sensor commercially available for clinical evaluations.
Several sensing mechanisms based on magneto-resistive, piezoelectric, piezoresistive, capacitive, opto-mechanical, and optical fiber sensors have been studied in the past. Some of these sensors, e.g. the magneto-resistive and piezoresistive strain sensors, have very complicated mechanical constructions and relatively large sizes. As a result, only a few sensors can be implemented in the insole to measure shear stress at critical locations. Other sensors, including the piezoelectric thin film, capacitive, and optical-based sensors, require sophisticated data conditioning and acquisition systems. Moreover, all of the conventional shear sensors require wire connections for power and data transmission. This not only limits the range of motion of the patients but also causes durability concerns if the sensors will be worn by the patients on a regular basis.